Light-emitting matrix array display devices with light sensing elements

ABSTRACT

A display device has an array of pixels ( 10 ) comprising light emitting display elements ( 20 ), for example EL elements, carried on a substrate ( 50 ) and associated light sensing elements ( 40 ) responsive to light emitted by the display elements. The light sensing elements each comprise a gated photosensitive thin film device such as a TFT structure or a lateral gated pin device having a semiconductor layer ( 52 ) with contact regions ( 53, 54 ) laterally spaced on the substrate and separated by a gate controlled region ( 55 ). A part of the associated display element ( 20 ) extends over the gate controlled region with an electrode ( 70 ) of the display element serving as the gate of the photosensitive device thereby ensuring good optical coupling between the display element and the photosensitive device and enabling the gate to be appropriately biased. Such an arrangement enables, for example, the provision of electro-optic feedback control in the pixel in comparatively simple manner.

[0001] This invention relates to light emitting matrix array displaydevices with light sensing elements. More particularly, the invention isconcerned with a matrix array display device comprising an array ofaddressable pixels comprising light-emitting display elements, and lightsensing elements. The invention is concerned especially, but notexclusively, with matrix display devices using electroluminescentdisplay elements, particularly organic electroluminescent displayelements, OLEDs, including polymer electroluminescent elements, PLEDs.

[0002] An example of matrix display device whose pixels compriseelectroluminescent (EL) display elements and light sensing elements isdescribed in British Application No 0005811.5. The described devicecomprises an active matrix display device having an array of pixelscarried on a substrate, in which each pixel includes a current-drivenelectroluminescent display element comprising light emitting EL materialbetween two electrodes, one of which is transparent, and a switchingdevice operable to control the current through the display element, andhence its light output, in a drive period based on a drive (data) signalapplied to the pixel in a preceding address period.

[0003] As in other active matrix EL display devices, such as the devicedescribed in EP-A-0717446, the display elements, which need tocontinuously pass current in order to generate a light output, can beenergised for an extended period, up to a frame time, following theaddressing of the pixel in a respective row address period with thelevel of the data signal stored in the pixel in the address perioddetermining its output during this drive period. The driving device, inthe form of a thin film transistor (TFT), is responsible for controllingthe current through the display element and the applied data signal isstored as a charge on a capacitance coupled to the gate of this driveTFT so that the operation of the TFT is dependent on the stored charge.

[0004] The pixels in the device of British Patent Application No0005811.5 further include a thin film photosensitive device, comprisinga (PiN) photodiode or a photo-responsive TFT coupled to the storagecapacitance that is arranged in operation of the pixel to be reversebiased and is responsive to light emitted by the pixel's display elementin the drive period so as to leak charge from the capacitance at a ratedependent on the display element's light output level. Thus, by virtueof the photo sensitive device, opto-electronic feedback is providedwhich progressively adjusts the operation of the drive TFT controllingenergisation of the display element during the drive period to reducethe current flow through the display element, and hence its lightoutput, by progressively discharging the capacitance (assuming it ischarged upon addressing). The proportion of the total available driveperiod for which the display element is energised is, therefore,dependent on, and regulated by, this feedback arrangement according tothe element's light output. In this way the integrated light output froma display element in a drive (frame) period can be controlled so as,inter alia, to counteract any effects of ageing or degradation in thedisplay element's electroluminescent material, particularly a reductionin light output level for a given drive current level, which can occurover a period of time of operation, and also to compensate for theeffects of voltage drops occurring in current carrying lines supplyingthe pixels.

[0005] Such a technique is valuable in achieving a high quality displayby ensuring that pixel light outputs can be constant and uniform overtime. However, the implementation of such a pixel circuit can beproblematic. The photocurrent generated by the photosensitive deviceneeds to be very small in order to appropriately control the TFT gatepotential over a frame period if the use of a large storage capacitanceis to be avoided. Also, the provision in each pixel circuit of the lightsensitive element using thin film technology ideally should not undulycomplicate fabrication while at the same time good optical couplingbetween the light-emitting display element and the light sensing elementneeds to be ensured.

[0006] According to the present invention, there is provided a lightemitting display device comprising on a substrate an array ofaddressable pixels each comprising a light-emitting display elementhaving a layer of light-emitting material with a transparent electrodeon one side thereof, and a light sensing element responsive to lightemitted by the display element, wherein each light sensing elementcomprises a gated photosensitive thin film device comprising asemiconductor layer having contact regions laterally spaced on thesubstrate and an intervening gate controlled region over whichdielectric material is disposed, and wherein a part of thelight-emitting display element extends over the dielectric material andthe gate controlled region such that the transparent electrode of thelight-emitting display element at that part serves as the gate of thephotosensitive device and light emitted by the light- emitting materiallayer is incident on the semiconductor layer.

[0007] With this arrangement, the provision of the light sensing elementis relatively uncomplicated while at the same time good optical couplingbetween this component and the light-emitting material of the displayelement is reliably ensured. Such an arrangement, therefore, is highlybeneficial when used, for example, in the kind of display devicedescribed in the aforementioned application No 0005811.5. The basicstructure of the gated photosensitive device is generally similar tothat of TFTs commonly used in matrix display devices, e.g. the drivingTFTs of the aforementioned display device. In this respect, the gatedphotosensitive device may comprise a TFT or a gated lateral (pin) diode.The provision of this component is, therefore, entirely compatible withthe thin film technology used for matrix display devices and the devicecan easily be formed simultaneously with such TFTs from common thin filmlayers. Because a part of the light-emitting element extends over thephoto device and the active region of the device is directly and closelyassociated with the light-emitting material, then the device is highlyresponsive to variations in light output from the light-emittingmaterial. Moreover, as a part of the electrode of the light-emittingelement is utilised for the gate of the photosensitive device, then thegate potential of this device, corresponding to the potential of thiselectrode in operation of the device, can conveniently be biasedappropriately during operation such that it behaves as a photosensitiveleakage device by virtue of photocurrents generated therein in responseto light incident from the light-emitting layer in the manner requiredfor this component when used as a charge adjusting device in the displaydevice of the aforementioned type.

[0008] While the invention is particularly beneficial in theimplementation of the particular kind of pixel circuit discussed above,it is envisaged that it can be used to advantage in other light emittingmatrix display devices in which pixels include a light sensing deviceresponsive to the light emission of the pixel's display element foranother purpose rather than being used in the particular mannerdescribed.

[0009] The light-emitting elements preferably compriseelectroluminescent elements, such as OLED or PLED elements. However, itis envisaged that the invention could be used to similar advantage indisplay devices using other kinds of light-emitting elements and notnecessarily in the case of the light sensing element being used as partof an electro-optic feedback arrangement in the manner described.

[0010] Embodiments of light-emitting display devices in accordance withthe invention, and in particular active matrix EL display devices, willnow be described, by way of example, with reference to the accompanyingdrawings in which:

[0011]FIG. 1 is a simplified schematic diagram of an embodiment of anactive matrix EL display device according to the present invention;

[0012]FIG. 2 shows the equivalent circuit of a few typical pixels in thedevice of FIG. 1;

[0013]FIGS. 3 and 4 are respectively plan and sectional schematic viewsof part of a pixel;

[0014]FIG. 5 is a schematic view through a part of an alternative formof pixel in a further embodiment; and

[0015]FIG. 6 shows the equivalent circuit of a typical pixel in thefurther embodiment.

[0016] The Figures are merely schematic. The same reference numbers areused throughout the Figures to denote the same or similar parts.

[0017] Referring to FIG. 1, the active matrix electroluminescent displaydevice comprises a panel having a row and column matrix array ofregularly-spaced pixels, denoted by the blocks 10, each comprising anelectroluminescent display element and an associated driving devicecontrolling the current through the display element, and which arelocated at the intersections between crossing sets of row (selection)and column (data) address conductors, or lines, 12 and 14. Only a fewpixels are shown here for simplicity. The pixels 10 are addressed viathe sets of address conductors by a peripheral drive circuit comprisinga row, scanning, driver circuit 16 and a column, data, driver circuit 18connected to the ends of the respective conductor sets.

[0018] Each row of pixels is addressed in turn in a frame period bymeans of a selection signal applied by the circuit 16 to the relevantrow conductor 12 so as to load the pixels of the row with respectivedata signals, determining their individual display outputs in a frameperiod following the address period, according to the respective datasignals supplied in parallel by the circuit 18 to the column conductors.As each row is addressed, the data signals are supplied by the circuit18 in appropriate synchronisation.

[0019]FIG. 2 illustrates the circuit of a few, typical, pixels. Eachpixel 10 includes a light emitting organic electroluminescent displayelement 20, represented here as a diode element (LED), and comprising apair of electrodes between which one or more active layers of organicelectroluminescent light-emitting material is sandwiched. In thisparticular embodiment the material comprises a polymer LED material,although other organic electroluminescent materials, such as lowmolecular weight materials, could be used. The display elements of thearray are carried, together with the associated active matrix circuitry,on the surface of an insulating substrate. The substrate is oftransparent material, for example glass, and the electrodes of theindividual display elements 20 closest to the substrate consist of atransparent conductive material such as ITO so that light generated bythe electroluminescent layer is transmitted through these electrodes andthe substrate so as to be visible to a viewer at the other side of thesubstrate. The cathodes of the display elements comprise a metal havinga low work-function such as calcium, a magnesium silver alloy, or abarium/aluminium dual layer. Examples of suitable organic conjugatedpolymer materials which can be used are described in WO 96/36959.Examples of other, low molecular weight, organic materials are describedin EP-A-0717446, which also describes the construction and operation ofa typical known form of active matrix electroluminescent device andwhose disclosure in these respects is incorporated herein by reference.

[0020] Each pixel 10 includes a driving device in the form of a lowtemperature polysilicon TFT 22, here of p-type conductivity, which isresponsible for controlling the current through, and hence operation of,the display element 20 on the basis of a data signal voltage applied tothe pixel. A data signal voltage for a pixel is supplied via a columnconductor 14 which is shared between a respective column of pixels. Thecolumn conductor 14 is coupled to the gate of the current-controllingdrive TFT 22 through an address TFT 26, also of p-type. The gates forthe address TFTs 26 of a row of pixels are all connected to a common rowconductor 12.

[0021] Each row of pixels 10 also shares a common voltage supply line 30held at a predetermined potential, and normally provided as a continuouselectrode common to all pixels, and a respective common current line 32.The display element 20 and the driving TFT 22 are connected in seriesbetween the voltage supply line 30 and the common current line 32 whichacts as a current source for the current flowing through the displayelement 20. The line 30, for example, may be at ground potential and theline 32 at a positive potential around, for example, 12V with respect tothe supply line 30. The current through the display element 20 isregulated by the drive TFT 22 and is a function of the gate voltage onthe TFT 22, which is dependent upon a stored control value determined bythe data signal.

[0022] An individual row of pixels is selected and addressed by the rowdriver circuit 16 applying a selection pulse to its associated rowconductor 12 which turns on the address TFTs 26 of the pixels anddefines a respective row address period. A data signal, in the form of avoltage level derived from the video information supplied to the drivercircuit 18 and applied to the column conductor 14 by the driver circuit18, is transferred by the address TFT 26 to the gate node 24 of thedrive TFT 22. At the end of the row address period the addresstransistor 26 turns off, and the voltage on the gate node 24 is retainedby a pixel storage capacitor 36 connected between the gate of the TFT 22and the common current line 32, so as to maintain the operation of thedisplay element during the subsequent drive period.

[0023] The voltage between the gate of the TFT 22 and the common currentline 32 determines the current passing through the display element 20,the current flowing through the display element being a function of thegate-source voltage of the drive TFT 22 (the source of the p-channeltype TFT 22 being connected to the common current line 32, and the drainof the TFT 22 being connected to the display element 20). This currentin turn controls the light output level (grey-scale) of the pixel. TheTFT 22 is biased as a current source and operates in saturation, so thatthe current flowing through the TFT is insensitive to the drain-sourcevoltage and dependent on the gate-source voltage. Consequently, slightvariations of the drain voltage do not affect the current flowingthrough the display element 20. The voltage on the voltage supply line30 is therefore not critical to the correct operation of the pixels.

[0024] Each row of pixels is addressed in turn in this manner in arespective row address panel so as to load the pixels of each row insequence with their respective drive signals and set the pixels toprovide desired display outputs during the subsequent drive period,corresponding approximately to a frame period, until they are nextaddressed.

[0025] In each pixel an opto-electronic arrangement is employed tocompensate for effects of display element degradation, whereby theefficiency of its operation in terms of the light output level producedfor a given drive current diminishes. Through such degradation displayelements that have been driven longer and harder will exhibit reducedbrightness, causing display non-uniformities. The opto-electronicarrangement counteracts these effects to an extent by controlling theintegrated, total, light output from an element in a drive periodaccordingly. The pixel circuits are similar in this respect to thosedescribed in British Patent Application No. 0005811.5 to which referenceis invited for a fuller description of such operation and whosedisclosure in this respect is incorporated herein by reference. Briefly,electro-optical feedback is used to adjust the charge on the storagecapacitor during the drive period by discharging the capacitor at a ratedependent on the instantaneous light emission of the display elementduring this period. Consequently, for a given data signal value thelength of time for which a display element is energised to generatelight during the drive period following the address period is regulatedaccording to the subsisting drive current/light emission levelcharacteristic of the display element, as well as the level of theapplied data signal, such that the effects of degradation, particularlywith regard to display non-uniformities, are reduced and the lightoutput from individual pixels can then be substantially the same aswould be obtained with a non-degraded display element if required.

[0026] Referring to FIG. 2 the electro-optic discharging means in thisdevice comprises a gated photo-sensitive thin film device 40, which hereis in the form of another TFT whose current-carrying, source and drain,electrodes are connected across the storage capacitor 36, to the gatenode 24 of the drive transistor 22 and the current line 32, and whosegate is connected to the node, 41, between the drive TFT 22 and thedisplay element 20. In this particular embodiment, where the drive TFT22 (and address TFT 26) comprises an p-type low temperature polysiliconMOS TFT, then the device 40 is of an opposite conductivity type, i.e an-type polysilicon MOS TFT.

[0027] As will be described in greater detail, the pixel is constructedand arranged in such a way that the gated photo-sensitive device 40 isexposed to light emitted by the display element 20 in operation of thepixel. At the end of the addressing phase a voltage is set on the gatenode 24 of the drive TFT 22, according to the level of the applied datasignal, and the capacitor 36, charged to this voltage level, serves tomaintain the gate voltage of the TFT 22, at least initially, in thesubsequent drive phase. The drain junction of the photosensitive device40 coupled to the line 32 is reverse biased and photo-responsive, andlight emitted by the display element in the drive period causes a smallphotocurrent to be produced in the device 40 which is approximatelylinearly proportional to the display element's instantaneous lightoutput level. The effect of this photocurrent is to slowly discharge thestorage capacitor 36, the amount of photocurrent, and thus the rate ofdischarge, being dependent on the light output level of the displayelement. The gate of the TFT 40 is positively biased, with its voltagecorresponding to the voltage at the node 41 and always zero or negativewith respect to the node 24, and always negative with respect to theline 32, and this ensures that the TFT 40 is held in its off (nonconductive) state. Accordingly, the transistor 40 behaves merely as aleakage device, in the manner of a reverse biased photodiode, whichcauses leakage of charge on the capacitor 36. The resultant dischargingof the capacitor 36 in the drive period leads to the voltage on the gateof the drive TFT 22 gradually reducing which in turn progressivelylowers the current flowing through the display element 20 with the lightoutput of the display element gradually decreasing in correspondingfashion, until the TFT 22 approaches its threshold, turn-off, level. Thereduction in current flowing through the display element 20 leads to agradual increase in the (positive) voltage level at the node 41,although this merely ensures that the TFT 40 is continuously held off.When, eventually, the voltage on the gate node 24 drops to below theTFT's threshold voltage, the light output is terminated. As examples oftypical voltages present in operation of the pixel, assuming for examplethat the TFT 22 has a −5 volt threshold, the voltage supply line 30 maybe at around 0 volts, the common current line 32 may be at 12 volts, andas the voltage at the gate node of the transistor 22 changes from 4 to12 volts the voltage at the node 41 can change from 4 volts to 0 volts.

[0028] By regulating the total, integrated, amount of light emitted bythe display element within the drive period, which a viewer perceives asbrightness, the effects of display element degradation can becounteracted. The integrated light output (brightness) is dependent onthe length of time in the drive period for which the display element isenergised as well as its initial light level. Because of the action ofthe discharging means in controlling the duration for which the displayelement is energised in the drive period, then different pixels in thearray supplied with the same data signal value will tend to producesimilar perceived brightness levels regardless of variations in thecharacteristics of their individual display elements due to degradation.In other words, the integral of the light outputs from individualdisplay elements addressed with the same data signal value will besimilar even though at the start of the drive period their respectivelight output levels may differ due to degradation effects. Improveduniformity of display output is thus obtainable.

[0029] As usual, the level of the applied data signal is adjustedappropriately to provide different grey-scale levels from the pixels. Ifthe data signal, and thus charge on the gate node 24, is increased thenmore photons are required from the display element during the driveperiod before the TFT 22 is caused to switch off, so that a highergrey-scale level is achieved, and vice versa.

[0030] This manner of operation is effective also to compensateautomatically for variations in the operational characteristics of theTFTs 22 of different pixels in the array resulting, for example, fromvariations in their threshold voltages, dimensions, and mobilities dueto the nature of the thin film fabrication processes used to form theTFTs. Thus, further improvement in the uniformity of light output fromthe display elements over the array is achieved.

[0031] Referring now to FIGS. 3 and 4, there are shown schematic planand sectional views through a part of a typical pixel including thephotosensitive TFT 40 and illustrating the manner of the pixelconstruction at this region. FIG. 4 corresponds approximately to asectional view along the lines IV-IV of FIG. 3. In addition to the TFT40, the part shown includes a portion of the display element 20 and thestorage capacitor 36, but does not encompass the addressing and driveTFTs 26 and 22. It will be appreciated though that these lattercomponents are fabricated together with the components shown using thesame processes and from common deposited layers.

[0032] On the transparent insulating substrate 50 a semiconductor island52 of elongate strip form and comprising a layer of low temperaturepolysilicon material is provided. This is obtained by laserrecrystallising a CVD deposited amorphous silicon layer andappropriately patterning this layer using a mask and photolithographicprocessing. The semiconductor strip is generally rectangular in shape,having substantially parallel major sides, and thus having substantiallyconstant width along its length. The opposing end portions of thisisland are doped (n+) to constitute laterally—spaced drain and sourcecontact electrode regions 53 and 54 respectively which are separated bya co-planar region of intrinsic semiconductor material 55 forming a gatecontrolled, channel, region of the TFT 40. Corresponding,similarly-shaped, islands of polysilicon are formed at other intendedpixel locations on the substrate at the same time and together withsemiconductor islands for the addressing and drive TFTs 26 and 22,although the regions of these latter TFTs constituting source and drainelectrodes are oppositely doped (p+type) instead. An insulating layer56, for example of silicon dioxide or nitride, is deposited continuouslyover the substrate to cover these islands and serve as a gate dielectriclayer.

[0033] A layer of metal, for example of aluminium, or an aluminiumalloy, is deposited over the layer 56 and patterned to leave regionsconstituting the gates (not shown) of the TFTs 26 and 22 and a region 58overlying the (drain) region 53 at each photosensitive TFT location. Atthe same time, required interconnection lines are formed from this metallayer. As is apparent from FIGS. 3 and 4, the region 58 is defined as arectangular finger or strip extending substantially transversely of thesemiconductor island 52. At the region of their cross-over, therefore,both the island 52 and the finger 58 are parallel-sided and ofsubstantially constant width. Together, the overlying portions of themetal finger 58 and the n+region 54 and the intervening portion of thedielectric layer 56 constitute the pixel's storage capacitance 36, whosecapacitance value is determined by the cross-over area between thefinger 58 and the island 52 and the thickness and dielectric constant ofthe layer 56.

[0034] Another dielectric layer 60, e.g of silicon oxide nitride, isformed over this structure covering, inter alia, the defined regions 58of the metal layer. A further metallisation layer is then deposited andpatterned to leave regions 62 forming the current lines 32 and otherrequired interconnections. Prior to depositing this layer, contactopenings 64 and 65 are formed by etching through both the dielectriclayers 56 and 60 over the source region 53 and the drain region 54 andcontact openings 66 are formed through the layer 60 over an end portionof the region 58 so that, following deposition and patterning of thismetal, interconnections are provided between the current line 32, via anintegral extension arm 67, and the drain electrode 53, between thesource region 54 and the gate node 24 via a part of the metallisation 62forming the current lines (through further contact openings which arenot shown), and between the current line 32 and the metal finger 58.

[0035] Transparent conductive material such as ITO is then deposited andpatterned to leave regions constituting lower (anode) electrodes for thedisplay elements which are appropriately shaped to define the desiredshape of the display elements. A portion of this electrode forms anintegral leg 70 which extends away from the main display element area 71(only a small part of which is shown) and transversely over thesemiconductor island 52 directly above the gate controlled region 55 andthe drain junction.

[0036] A further, relatively thick and continuous, dielectric layer 73e.g of silicon nitride or an even thicker (1-2 μm) insulating polymerlayer is deposited completely over the structure and openings 74 areformed in this layer above the patterned ITO regions both at the legs 70and main display element areas.

[0037] The polymer light emitting material is then deposited, forexample by spin coating, as a continuous layer 80 extending over thedielectric layer 73 and into the openings 74 formed therein so as tocontact directly with the underlying ITO. Over this layer 80, acontinuous layer 82 of calcium, magnesium silver alloy orbarium/aluminium is deposited to form a common electrode layerconstituting the display element cathode electrodes and the supply lines30.

[0038] Each display element 20 consists of a respective region 71 of ITOtogether with overlying portions of the layers 80 and 82 and it will beappreciated that the integral ITO leg 70 together with the immediatelyoverlying parts of the layers 80 and 82 form an integral extension ofthe display element which with the main display element region emitslight upon a suitable potential difference being applied between thebottom and top electrodes.

[0039] The portion of the ITO leg 70 immediately overlying the gatecontrolled region 55 serves as the gate of the photosensitive TFT 40with the underlying combined layers 56 and 60 providing the gatedielectric.

[0040] In operation of the pixel, light emitted by the layer 80 uponcurrent being passed between the electrodes 71 and 82 is transmittedthrough the ITO lower electrode and the substrate 50 to produce adisplay output. The leg of the display element similarly produces lightand this passes through the ITO extension 70 and the underlying,transparent, dielectric layers 56 and 60 so as to be incident on thegate controlled region 55 of the photosensitive TFT 40. The lightfalling on the drain junction particularly is responsible forphotocurrent being generated. Thus, as a result of the display elementleg extending over the region 55, and the light emitting polymermaterial 80 being immediately above the gate of TFT 40 and directlyemitting light into the TFT structure, good optical coupling between thedisplay element and the photosensitive TFT 40 is ensured and achieved insimple and reliable manner.

[0041] Moreover, as the gate of the TFT 40 is constituted by a portionof the display element anode, then the gate is always at the required(negative) bias with respect to both source and drain to ensure that theTFT 40 is held off (i.e in its high resistance, non-conducting, state)and that only leakage currents due to generated photocurrents flowbetween its source and drain electrodes.

[0042] The relationship between the photosensitive TFT 40 and thestorage capacitance 36 in terms of the level of photocurrent generatedin response to typical input light levels and the amounts of chargestored on the storage capacitor 36 needs to be closely controlled inorder for the electro-optic feedback control to be implemented mosteffectively. The active area of the TFT 40 in this respect comprises theedge of a lateral (n+−i) drain junction and it is only thiscomparatively narrow area at the drain junction which contributes aphotocurrent. The active area is basically equivalent to a photodiodeand desirably should be very small in order to ensure the typical levelsof photocurrent generated are sufficiently low to control the gatepotential of the drive TFT 22 in the manner required over the drive(frame) period and avoid the need to use a larger storage capacitor.Because the storage capacitor 36 uses in its structure only the layer 56as the capacitor dielectric then the area required for the capacitorstructure to provide a given capacitance value is much smaller thanwould be the case if both the layers 56 and 60 were used.

[0043] When using thin film technology it can be difficult to formcomponents with very precise dimensional values because of the nature ofthe processes employed to define the components, e.g. the masking andetching steps used in photolithographic patterning processes. It will beappreciated that in the above-described structure the storage capacitor36 and the photosensitive TFT 40 both share the same critical layer,namely the semiconductor island 52 and are spatially close. Accordingly,any line width variations in this part due to manufacturing toleranceswill be common to both. Because this critical geometry, i.e the width ofthe strip-shape island 52 as indicated by X in FIG. 3, is constant forboth the TFT 40 and the storage capacitor then any deviation in thisphysical dimension of this common part from the intended value willaffect both the active area of the TFT and the capacitance value of thestorage capacitor in a similar, corresponding fashion. More precisely,the active area of the TFT and the capacitance value scale together. Anyvariation in the width of the island 52 tending to increase the size,and hence capacitance, of the storage capacitor will result in the sizeof active area of the TFT also being increased, and vice versa, so thatthe balance between the electrical characteristics of these twocomponents will be maintained. Thus, the desired, and predetermined,inter-relationship of these two components is ensured.

[0044] A gated lateral p-i-n diode may conveniently be used in place ofthe TFT 40. The structure of such a device would be generally similar tothat shown in FIG. 3 except that the region 53 of the semiconductorisland 52 would be doped oppositely to the region 54, i.e. p+ type. Inreverse bias, the p+ region 53 is more positive than the n+region 54. Inthis case, light incident across the whole device can generatephotocurrent. The photo-active junction thus will extend to a greaterdistance across the device than is the case with the previous, TFT,structure.

[0045] In the above described pixel embodiment, the photosensitive TFT40 is of opposite conductivity type to both the address TFT 26 and thedrive TFT 22, and by virtue of the manner in which the gate of the TFT40 is biased it is ensured that the TFT 40 is held off and acts simplyin the manner of a reverse-biased photodiode to leak charge on thecapacitor 26 during the drive period.

[0046] However, in an alternative form of this pixel circuit, the TFT 40may be of the same type as the drive TFT 22 and operable also as aswitching device rather than solely a leakage device. In operation ofthis alternative circuit, then initially at least in the drive periodthe gate potential, corresponding to the display element lowerelectrode/node 41 potential, will be such as to ensure that the TFTbehaves as a simple, reverse-biased, leakage device. As this dischargingcontinues then the consequential reduction in current flowing throughthe display element will lead to a gradual increase in the (negative)voltage level at the node 41 (FIG. 2). When the current level attains acertain lower limit, the voltage at the node 41 relative to the line 32will reach the threshold voltage of the TFT 40 causing it to turn on(conduct) abruptly and rapidly discharge the capacitor 26 so that thedrive TFT 22 turns off and energisation of the display element isterminated. The switching operation of the TFT 40 in this manner has theadvantage that the light output of the display element is determined ina more precisely controller manner. Without such switching, the turningoff of the display element could be less well controlled due to the factthat the behaviour of the photosensitive TFT 22 in response tocomparatively low light input levels, as would occur towards the end ofthe display element's energisation phase, becomes less well defined andless predictable.

[0047] The photosensitive TFT 40 desirably is shielded from the effectsof ambient light falling thereon so that any photocurrent is due solelyto light emitted from the display element. To this end, the metalelectrode layer 82 serves to shield the device from ambient light at oneside of the panel. Shielding of light from the other side, through thetransparent substrate 50, could be achieved by depositing a light shieldlayer between the semiconductor island 52 and the substrate surface.

[0048] Preferably, however, in another embodiment of device according tothe invention, the structure of the pixel is modified so that a part ofthe storage capacitor structure is employed also to act as a lightshield. FIG. 5 illustrates schematically a section through a part of themodified pixel structure for comparison with FIG. 4. In this structure,a metal layer is deposited on the surface of the substrate 50 andpatterned to form at each pixel a light shield 90 whose overalldimensions are slightly greater than those of the subsequently formedsemiconductor island 52 used for the TFT 40 (or alternatively a lateralpin diode). An insulating layer 92, for example of silicon nitride, isdeposited as a continuous layer completely over the substrate surfaceand those metal layers 90 to form a planar surface upon which the pixelstructure is then fabricated generally as previously described but inthis case with the metal finger 58 previously used being omitted andwith the portion of the metal layer 62 used to contact the region 53 ofthe semiconductor island 52 being arranged also to contact one end ofthe light shield layer 90 through the dielectric layers 60 and 92 via acontact hole formed in these layers away from the island 52. Thedielectric layer 56 is not necessary in this version.

[0049] In this construction, the storage capacitor 36 is formed by thepart of the light shield layer 90 overlying contact region 54 of theisland 52 together with the portion of the insulating layer 92sandwiched therebetween which provides the dielectric. The equivalentcircuit of this pixel is shown in FIG. 6. The photosensitive TFT 40 inthis case effectively has a double gate, with the light shield 90constituting a bottom gate. This second gate will be positive withrespect to the channel 55 and so the insulating layer 92 is required tobe sufficiently thick to ensure that the TFT's threshold level is notreached and that the TFT is prevented from being turned on.

[0050] Instead of overlying the island 52 completely as in the abovearrangement, the layer 90 may be configured such that it adequatelycovers the photo-active region 53 of the TFT 40 for light shieldingpurposes by extending completely over the region of the drain junctionand over the source region 54 to form the capacitor but does not extendto any significant extent over the channel region. To this end, thelayer 90 may have a central aperture overlying the channel region 55bounded and by integral arms extending parallel to the island to eitherside of the channel region so that the regions of the layer 90 overlyingthe two contact regions are interconnected. With this arrangement, thenbecause the layer does not extend directly over the channel region 55the risk of it acting as a second gate is avoided.

[0051] In this other embodiment, then again the TFT 40 and storagecapacitor 36 both share the same critical layer, i.e the island 52, sothat any spatial variation due to fabrication process tolerances arecommon to both components and they scale together.

[0052] The invention can be used also with pixels driven using a currentdata signal rather than a voltage data signal as in the above-describedembodiments, for example in the manner described in WO 99/65012.

[0053] In summary, a display device has an array of pixels comprisinglight emitting display elements, for example EL elements, carried on asubstrate and associated light sensing elements responsive to lightemitted by the display elements. The light sensing elements eachcomprise a gated photosensitive thin film device such as a TFT structureor a lateral gated pin device having a semiconductor layer with contactregions laterally spaced on the substrate and separated by a gatecontrolled region. A part of the associated display element extends overthe gate controlled region with an electrode of the display elementserving as the gate of the photosensitive device thereby ensuring goodoptical coupling between the display element and the photosensitivedevice and enabling the gate to be appropriately biased. Such anarrangement enables, for example, the provision of electro-opticfeedback control in the pixel in comparatively simple manner.

[0054] From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the field of active matrixelectroluminescent display devices and component parts therefor andwhich may be used instead of or in addition to features alreadydescribed herein.

1. A light-emitting display device comprising on a substrate an array ofaddressable pixels each comprising a light-emitting display elementhaving a layer of light-emitting material with a transparent electrodeon one side thereof, and a light sensing element responsive to lightemitted by the display element, wherein each light sensing element is agated photosensitive thin film device comprising a semiconductor layerhaving contact regions laterally spaced on the substrate and anintervening gate controlled region over which dielectric material isdisposed, and wherein a part of the light-emitting display elementextends over the dielectric material and the gate controlled region suchthat the transparent electrode of the light-emitting display element atthat part serves as the gate of the photosensitive device and lightemitted by the light-emitting material layer is incident on thesemiconductor layer.
 2. A light-emitting display device according toclaim 1 , characterised in that the gated photosensitive devicecomprises a thin film transistor having laterally—spaced source anddrain contact regions of doped semiconductor material and a region ofintrinsic semiconductor material extending between the contact regionsforming the gate controlled region.
 3. A light-emitting display deviceaccording to claim 1 , characterised in that the photosensitive devicecomprises a gated pin diode device having laterally spaced contactregions of oppositely doped type semiconductor material and a region ofintrinsic semiconductor material extending between the contact regionsforming the gate controlled region.
 4. A light-emitting display deviceaccording to any one of claims 1 to 3 , characterised in that thetransparent electrode of the light-emitting element is arranged inoperation of the device to be biased such that current flow in the gatecontrolled region is due to generated photocurrents.
 5. A light-emittingdisplay device according to claim 4 , characterised in that the displayelement is current driven and current through the display element in adrive period is controlled by a thin film transistor based on a drivesignal applied to the pixel during a preceding address period and storedas a charge on a storage capacitor coupled to the gate of thetransistor, and in that the photosensitive device is coupled to thestorage capacitor and acts as a charge leakage device in response tophotocurrents generated therein to regulate the charge on the storagecapacitor in accordance with the display element's light output.
 6. Alight-emitting display device according to claim 5 and claim 2 ,characterised in that the photosensitive device is operable as a switchand arranged to turn on in response to the potential level of thetransparent electrode of the display element in the driving periodreaching a threshold level to provide a conductive path between itscontact regions and rapidly discharge the storage capacitor.
 7. Alight-emitting display device according to any one of the precedingclaims, characterised in that the light-emitting material of the displayelements comprises electroluminescent material.